This invention generally relates to variable valve actuation mechanisms for internal combustion engines and, more particularly, to an output cam for use with a variable valve mechanism.
A conventional internal combustion engine utilizes an air throttling device and a timing device. The throttle device is typically a valve that, in response to driver input, regulates the flow of air to the engine intake valves. The timing device includes a crankshaft that drives a rotary, lobed camshaft. Engine intake valves are opened and closed at predetermined angles of crankshaft rotation to allow the descending piston to draw air into the combustion chamber. The shape or lift profile of the cam lobes, in part, fixes the crankshaft angle at which the valves open/close and the amount by which the valves are lifted. The plot of valve lift relative to crankshaft angular position is referred to as a valve lift profile. A conventional engine has an intake valve lift profile that is generally parabolic in shape.
A modern internal combustion engine may incorporate a more advanced throttle control system, such as, for example, an intake valve throttle control system. An intake valve throttle control system, in general, controls the flow of gas and air into the cylinders by varying the timing and/or the amount of intake valve lift. The timing and/or amount of lift is varied dependent upon and in response to engine operating parameters, such as, for example, engine load, speed, and driver input. Intake valve throttle control systems vary the valve lift profile through the use of various mechanical and/or electromechanical configurations, generally referred to herein as variable valve actuating (VVA) mechanisms. One example of a VVA mechanism is detailed in commonly-assigned U.S. Pat. No. 5,937,809, the disclosure of which is incorporated herein by reference.
Conventional VVA mechanisms typically include an output cam lobe that is pivotally oscillated through a predetermined and fixed range of motion. The pivotal motion of the output cam lobe is transferred to opening and/or closing of a corresponding valve. More particularly, the output cam typically engages a roller finger follower that, in turn, engages a corresponding intake valve. The shape or lift profile of the pivoting output cam lobe causes a corresponding displacement or pivot of the roller finger follower and, in turn, a corresponding actuation or lifting of the intake valve. The amount and timing of the valve lift is varied by changing the angular position of the output cam lobe relative to the roller finger follower and/or a central axis of the cam lobe such that the roller finger follower is engaged by a desired portion of the output cam lift profile as it is pivoted.
For example, to impart a large amount of lift to the intake valve, the angular position of the output cam lobe relative to the roller finger follower is established such that the nose or peak of the lift profile is disposed within the fixed range of motion of the output cam. Thus, as the output cam lobe is pivoted through its fixed range of motion the peak of the lift profile engages the roller finger follower thereby actuating or lifting the valve a corresponding and relatively large amount. Conversely, to achieve a small amount of or zero lift the angular position of the output cam lobe relative to the roller finger follower is established such that the roller finger follower is engaged primarily or only by the base circle of the lift profile as the output cam lobe pivots through its fixed range of motion. Thus, the roller finger follower is pivoted and the corresponding valve is actuated a relatively small or zero amount.
Conventional VVAs vary the amount and timing of valve lift in order to, for example, increase engine power, reduce pumping work and/or improve charge preparation. The output cams of such VVAs incorporate conventional lift profiles. Since only the amount and/or timing of the valve lift is varied, the valve lift profiles remain generally parabolic in shape. The amount of valve lift in a given engine is fixed not only by the lift profile of the output cam lobe but also by the valve springs and other valve train components. The limited available or maximum amount of valve lift, in turn, limits the amount of air flow/intake and thereby limits engine power. The limited maximum lift also limits the resolution of, i.e., the difference in lift between, the valve lift profiles. The peak valve lift achieved by the lower-lift profiles must be a certain amount less than the maximum lift.
Therefore, what is needed in the art is an output cam for use with a VVA mechanism that extends the duration of the valve event.
Furthermore, what is needed in the art is an output cam for a VVA mechanism that increases air flow/intake for a given amount of valve lift.
Still further, what is needed in the art is an output cam for a VVA mechanism that increases engine power for a given amount of valve lift.
Moreover, what is needed in the art is an output cam for a VVA mechanism that increases the resolution of valve lift profiles by providing lower lift curves that represent a larger percentage of peak or maximum valve lift.
The present invention provides an output cam for a variable valve mechanism.
The invention comprises, in one form thereof, a body configured for being pivotally associated with and driven by an input shaft. A lift profile of the output cam includes a base circle portion, a cam portion and a fixed radius portion. The base circle portion is adjacent to and continuous with the cam portion, and the cam portion is adjacent to and continuous with the fixed radius portion. The base circle portion has a base radius, the cam portion has a cam radius and the fixed radius portion has a fixed radius. The base and fixed radii are substantially constant. The fixed radius is a predetermined amount greater than the base radius. The cam radius increases from a value approximately equal to the base radius adjacent the base circle portion to a value approximately equal to the fixed radius adjacent the fixed radius portion.
An advantage of the present invention is that a longer duration lift event is achieved.
A further advantage of the present invention is that engine power is increased for a given amount of valve lift.
A still further advantage of the present invention is that low-lift valve lift profile resolution is improved.
An even further advantage of the present invention is that longer duration lift events are achieved without requiring modifications to associated valve train components.
Yet further, an advantage of the present invention is that the peak or maximum amount of valve lift can be reduced without sacrificing power or air intake/flow.